Questions: Sedimentary Geochemistry

Question 1

A black shale has extremely high Mo (100 ppm) and U (20 ppm) concentrations compared to average shale (Mo ~1 ppm, U ~3 ppm). What does this indicate about the depositional environment?

• A. The shale was deposited in a highly oxidizing environment
• B. The shale was deposited under euxinic (anoxic, sulfidic) conditions where Mo is quantitatively scavenged from seawater by reaction with dissolved H2S and organic matter, and U(VI) is reduced to insoluble U(IV) and concentrated in the sediment
• C. The shale has been hydrothermally altered
• D. The high Mo reflects volcanic ash input

Answer: (B)

Mo is soluble in oxygenated seawater as molybdate (MoO4 2-) but is quantitatively removed under euxinic conditions where H2S converts it to particle-reactive thiomolybdate species that are scavenged. Similarly, U is soluble as U(VI) but reduced to insoluble U(IV) in anoxic waters. Extreme enrichments of both elements together are diagnostic of euxinic bottom waters -- a key indicator used to reconstruct ocean redox through Earth history.

Question 2

The Chemical Index of Alteration (CIA) can be used to assess the intensity of chemical weathering recorded in sedimentary rocks.

• T. True
• F. False

Answer: True

CIA = [Al2O3 / (Al2O3 + CaO* + Na2O + K2O)] x 100, where CaO* is silicate-bound CaO only. During chemical weathering, Ca, Na, and K are progressively leached from feldspars while Al remains in residual clay minerals. Fresh feldspar-rich rocks have CIA ~50; intensely weathered residual clays approach CIA ~100. Sedimentary rock CIA values between 50 and 100 record the cumulative intensity of source-area weathering, providing a proxy for paleoclimate (warm, humid climates produce more intense weathering).

Question 3

Explain why redox-sensitive trace elements are more reliable paleoenvironmental indicators than sediment color alone.

Model answer: Sediment color is qualitative, can be modified by diagenesis (organic matter oxidation, secondary iron reduction), and depends on mineral type as much as redox conditions. Redox-sensitive elements (Mo, U, V, Re) have quantitative enrichment patterns that are directly tied to dissolved-oxygen and sulfide concentrations in bottom water through well-understood aqueous chemistry. Their enrichment factors can be calibrated against modern environments with known redox conditions. Multi-element ratios (e.g., Mo/U) discriminate between suboxic, anoxic, and euxinic conditions. These geochemical proxies provide reproducible, quantitative redox reconstruction that sediment color cannot.

Color is ambiguous and alterable; element enrichments are quantitative and mechanistically linked to specific redox thresholds.